To explore the possibility that THE (tick-borne encephalitis) virus may become established in eastern North America where the agent of Lyme disease is enzootic, we shall compare the transmissibility of THE virus and of the Lyme disease spirochete in nature and in the laboratory. We shall (1) determine the prevalence of both infections in vector and reservoir hosts in intensively studied Eurasian field sites in which these agents coexist; (2) correlate vector abundance with the prevalence of spirochetal and of viral infection in the vector; (3) measure the prevalence of Babesia microti and of Ba. divergens in vector ticks; (4) explore the possibility of using CR technology-for diagnosing these various infections in place of conventional methods; (5) compare the virus competence of such potential vector ticks as Ixodes dammini, I. persulcatus and I. ricinus; (6) measure the duration of THE infectivity in the white-footed mouse, the main reservoir of the spirochete in eastern North America; (7) screen a battery of sera from Lyme disease patients residing in various New England sites; (8) extend a mathematical model now being developed to represent Ixodes-borne infection in eastern North America to include disease relationships in Eurasia. Taken together, we anticipate that these comparisons of the force of transmission and determinations of host competence will lead toward an understanding of the level of risk of human THE infection in American sites infested by I. dammini, the vector of the Lyme disease spirochete.